For the purpose of treating suspensions, in particular to comminute their solids fraction, grinding systems are known in which the suspension to be treated is circulated via a dispersing unit and a ball mill. Via a feed pump which is inserted, at a suitable position, into the conduit connecting the dispersing unit to the ball mill, the suspension is circulated within said circulation arrangement. Such grinding systems are employed, for example, to process paints and pigment paste.
Further examples of grinding systems designed for batchwise treatment of solids in liquids comprise a stirred mill unit which, by means of a hydraulically actuated lifting column, can be lowered into a vessel holding the suspension to be treated and whose constructional design corresponds to the known dissolvers. Via the rotation of a perforated basket, the suspension is subjected to a circulatory effect, and said product vessel may, for the purpose of keeping within temperature limits, be designed to be coolable, i.e. of double-walled design. Moreover it is possible, by virtue of a sealed vessel cover, to achieve substantially low-emission comminution. Such stirred mills are known, for example, from the "NETSCH-Turbomill" brochure from NETSCH.
These known grinding systems are distinguished by a relatively complex construction, or the grinding process is carried out, in part, in an open system. A further aspect in some cases is an inadequate circulatory effect and--related thereto--a reduced grinding action. This deficiency becomes all the more important in systems involving difficult rheology. For example, certain systems, when being pumped, show a tendency to solvent depletion, which causes caking, blockages and sedimentation.
DE 295 18 987 U1 discloses a dispersing apparatus which comprises a stirred ball mill which is positioned in a height-adjustable manner within a vertically disposed cylindrical vessel and underneath which--in a fixed height position relative to the vessel--a flow generation apparatus in the form of a dissolver is installed. The stirred ball mill comprises a housing which is perforated like a sieve and is in the form of a toroidal annular duct which extends coaxially with respect to the vessel axis and whose outer periphery is maintained at a distance from the inside of the housing, said annular duct enclosing a central opening through which runs the dissolver drive shaft, which extends in the axial direction of the vessel. Said drive shaft is guided, at the top end of the vessel, in a tubular shaft by means of which the agitator, which is situated within the stirred ball mill and is formed by a system of annular disks, can be driven. The agitator on the one hand and the dissolver on the other hand can be driven by means of drives situated outside the vessel--alternatively, however, a common drive for both apparatuses can be provided. Apart from this, the stirred ball mill is suspended on rods within the vessel, and a further driving means is provided for adjusting the height. Whereas the dissolver effects predispersion of the material to be dispersed, with the stirred ball mill in a raised position, i.e. outside the material to be dispersed, lowering the stirred ball mill achieves not only a grinding action but also achieves fine dispersion. In the lowered state, the sieve-like housing of the stirred ball mill is immersed into the material to be dispersed, which has been set into a rotary flow motion by the dissolver, the field of flow which is being established forming a circulation which partially permeates said housing. The advantage of this known apparatus is that said circulation is established entirely within the reaction vessel. The emphasis, however, is on the dispersing operation, and the achievable grinding action depends on the particular way of guiding the flow within the circulation.